This invention relates to devices which detect and/or treat tachyarrhythmias (rapid heart rhythms), and more specifically, to stabilization of the ventricular rate during atrial fibrillation.
In the context of dual chamber pacing, a variety of mode switching features have been developed which detect an excessively rapid atrial rhythm and in response cause the pacemaker to switch from an atrial synchronized pacing mode such as DDD to a non-synchronized mode such as VVI or DDI. Such mode switching features are disclosed in U.S. Pat. No. 5,144,949, by Olson, U.S. Pat. No. 5,318,594, by Limousin et al., U.S. Pat. No. 4,944,298, by Sholder, U.S. Pat. No. 5,292,340, by Crosby et al. and U.S. Pat. No. 4,932,406 by Berkovits, all incorporated herein by reference in their entireties. In such devices, the primary purpose of the mode switch is to prevent the pacemaker from tracking a non-physiologic atrial rate. Unfortunately, these mode switching mechanisms do not address the problem of inappropriately high, irregular ventricular rhythms which often result, particularly in the presence of atrial fibrillation.
As set forth in the article "Rate Stabilization by Right Ventricular Pacing in Patients With Atrial Fibrillation", by Wittkampf et. al., published in PACE, Vol.9, Nov.-Dec., 1986, Part II, pp 1147-1153, these rapid, variable ventricular rhythms have negative hemodynamic consequences. The Wittkampf article therefore proposes VVI pacing with a self adapting pacing rate selected to result in 91% of all heartbeats being paced. The article states that this methodology provides stable ventricular rates in the presence of atrial fibrillation with only a moderate increase in over-all rate. A dual chamber pacemaker which addresses the same problem is disclosed in U.S. Pat. No. 5,480,413, issued to Greenhut et al. This device detects the presence of atrial tachyarrhythmia and a concurrent irregular ventricular heartbeat and raises the ventricular pacing rate until the ventricular rhythm is regularized. Both of these prior art approaches operate by analyzing a series of preceding ventricular heartbeats and gradually modulating the pacing rate until a stable ventricular rhythm is accomplished.
U.S. Pat. No. 4,971,471, by Mehra et al and U.S. Pat. No. 5,545,185, by Denker disclose pacemakers designed to prevent the short-long heartbeat interval patterns accompanying PVC's, which are often associated with the onset of ventricular tachyarrhythmias. These pacemakers, unlike those in the Wittkampf and Greenhut references, modulate the pacing interval on a beat by beat basis to provide pacing interval slightly longer than a preceding intrinsic interval. In the Mehra patent, the pacing modality is disclosed as continuously activated. In the Denker patent, the pacing modality is activated only on detection of a long-short interval pattern.